Catena 40 Ž2000. 51–68
www.elsevier.comrlocatercatena
The effect of land use change on soils and
vegetation over various lithological formations on
Lesvos žGreece /
C. Kosmas ) , St. Gerontidis, M. Marathianou
Laboratory of Soils and Agricultural Chemistry, Agricultural UniÕersity of Athens, Iera Odos 75, Botanikos,
Athens 11855, Greece
Abstract
The effect of land use change, from arable to abandoned pasture, on soil properties and
vegetation establishment was studied in hilly areas of the island of Lesvos. The main purpose of
this study was to establish key indicators or land parameters that can be used for defining
environmentally sensitive areas to desertification. 106 soil sites were selected in fields cultivated
or non-cultivated for 40–45 years and measurements related to protection from land degradation
after abandonment, such as fertility status Žorganic matter content, pH, cation exchange capacity
ŽCEC., exchangeable potassium and sodium., water storage capacity Žsoil water retention characteristics, soil depth., erosion resistance Žsoil aggregate stability., and vegetation characteristics
Žplant species, extent of vegetation cover. were conducted. The selected sites were located on a
variety of parent materials such as volcanic lava, pyroclastics, ignimbrite, schist-marble, and shale.
The obtained data indicated that soil pH, and CEC were slightly affected after abandonment as
compared to the cultivated soils. Levels of exchangeable sodium and potassium were higher in
cultivated soils. Organic matter content and soil aggregate stability were greatly enhanced in most
of the abandoned soils. Physical characteristics of the parent material greatly influenced the
establishment of the natural vegetation. A critical minimum soil depth of 25–30 cm was
measured. On more shallow soils, the associated natural perennial vegetation cover was rapidly
reduced, under the prevailing climatic conditions of the study area. Reduction of vegetation cover
was related to the parent material. Perennial vegetation was not supported at all on soils below a
crucial depth, ranging from 4 to 10 cm, depending on the parent material. This study showed that
soil depth is the most important parameter which has to be considered in planning land use change
)
Corresponding author. Fax: q30-1-5294097
0341-8162r00r$20.00 q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 4 1 - 8 1 6 2 Ž 9 9 . 0 0 0 6 4 - 8
52
C. Kosmas et al.r Catena 40 (2000) 51–68
from arable to unmanaged pasture under the soil and climatic conditions of the study area. q 2000
Elsevier Science B.V. All rights reserved.
Keywords: Land use change; Soil properties; Soil depth; Parent material; Desertification
1. Introduction
The particular nature of the typical Mediterranean relief, with slopes subject to
intensive cultivation since ancient times, has led to soil erosion and the formation of
shallow skeletal soils. As the soil is eroded, land use is usually shifted from agriculture
to pasture due to increasingly poor yields from the various agricultural crops. Most
pasture land in the Mediterranean region is defined as abandoned land today ŽMartinezFernandez et al., 1995; Lopez-Bermudez
et al., 1996; Puigdefabregas et al., 1996; Roxo
´
´
et al., 1996.. Previous authors have simultaneously used the terms ‘abandoned land’ and
‘grazing land’, but grazing or hunting of an abandoned land is considered as a traditional
use in the Mediterranean region. Only a few areas which have been fenced or strictly
controlled by land owners remain ungrazed. Almost all the natural vegetation in the
Mediterranean basin, apart from a few forests, is grazed to some extent by migrating or
permanent flocks and herds ŽClark, 1996.. Therefore, the term ‘abandoned land’ is used
in this study to include areas previously cultivated but now abandoned, and the natural
vegetation has been allowed to grow under various intensities of grazing.
The effects of land abandonment on land quality may be positive or negative
depending on the soils and climatic conditions of the area. Soils under favourable
climatic conditions that sustain plant cover may improve with time by accumulating
organic materials, increasing floral and faunal activity, improving soil structure, increasing infiltration capacity, and therefore, decreasing erosion potential ŽTrimble, 1990..
Martinez-Fernandez et al. Ž1995. have reported a positive effect of land abandonment
after a period of over 10 years in which characteristics of abandoned soils approached
those seen before cultivation. The abandonment of this area resulted in improvement of
soil characteristics such as the organic matter content, water retention capacity, aggregation and structural stability. Jaiyeoba Ž1995. and Unger Ž1997. reported a deterioration
of soil fertility under cropping and concluded that the soils under various types of
agricultural land uses contained less organic matter content, total nitrogen, exchangeable
bases and cation exchange capacity ŽCEC. than similar soils under natural vegetation.
Greenland and Nye Ž1959., Aweto Ž1981., Jaiyeoba Ž1988., and Ekanade Ž1989. showed
that under forest fallow soil organic matter content and nutrients increased. Lopez´
Bermudez
et al. Ž1996., in a study along a hillslope under various conditions of
´
abandonment and cultivation in southern Spain, showed that soils on fallow, or
abandoned for 4–10 years, showed a progressive recovery of vegetation cover and a
significant increase in organic matter content, aggregate stability, water holding capacity
ŽWHC., and hydraulic conductivity.
Where vegetation cover is sparse, erosional processes may be very active and the
degeneration of the abandoned lands may be irreversible. Many authors have demonstrated that in a wide range of environments both runoff and sediment loss decrease
C. Kosmas et al.r Catena 40 (2000) 51–68
53
exponentially as the percentage of vegetation cover increases ŽElwell and Stocking,
1976; Lee and Skogerboe, 1985; Francis and Thornes, 1990.. Garcia Ruiz Ž1991. found
that only 3.5% of fields abandoned for a period of less than 10 years in the Spanish
Pyrenees did not suffer from erosion, and 60% of the fields suffered severe sheet erosion
without sufficient protection from a vegetation cover. Kosmas et al. Ž1997. showed that
in general on hilly Mediterranean shrublands runoff and sediment loss increased with
decreasing annual rainfall Žabove a threshold of 280–300 mm., and attributed this to a
decrease in vegetation cover. For areas with rainfall below this threshold, erosion
decreased with increasing rainfall. Martinez-Fernandez et al. Ž1996. demonstrated that
the post-abandonment land use is of major importance for the evolution of favourable
land characteristics after abandonment.
A major aim in studying the ability of an ecosystem to return to the original state
after disturbance Žresilience. is the prediction of the response of such a system to a
variety of natural and human-induced disturbances ŽDell et al., 1986; Westman, 1986..
These include drought, fire, grazing, vegetation clearing and cultivation. Moderate
grazing pressure on abandoned agricultural land may lead to partial rejuvenation of
vegetal communities with a high diversity index ŽFox and Fox, 1986; Martinez-Fernandez et al., 1996.. The decline in vegetation by overgrazing can include the loss of
particular herbaceous families ŽLeguminosae, Gramineae. which help to maintain soil
structure. Plant species from these families can both protect the soil surface from
raindrop splashing and reduce erosion rates by increasing soil aggregate stability.
Disturbance by grazing does not result in the complete removal of vegetation as an
intense fire does. The impact of fire is greatest in those areas with the lowest fire
frequencies ŽFox and Fox, 1986.. An increase in fire frequency leads to fewer plant
species, caused by the loss of those which cannot persist when fires are too frequent
ŽFox and Fox, 1986; Grove and Rackham, 1996..
Parent material is considered as a soil-forming factor affecting soil properties, plant
growth ŽKosmas et al., 1993., soil erosion and ecosystem resilience. Hilly soils formed
on consolidated parent materials such as limestone, sandstone, volcanic lava, etc. usually
have a restricted effective rooting depth as the soils are eroded and shallow. Soil depth
defines the root space and the volume of soil from where the plants fulfil their water and
nutrient demands. Under hot and dry climatic conditions, rain-fed vegetation may not be
supported in such areas, leading to desertification. The number of studies on the effect
of parent material on regeneration of natural vegetation and protection of abandoned
agricultural land are limited. Furthermore, there are few studies which address the issue
of when a land has to be abandoned under certain soil and climatic conditions and how
sufficient regeneration of natural vegetation and protection of the soil from erosion
develop. An attempt has been made in this study to evaluate the effect of parent material
on soil evolution and vegetation recovery following abandonment of agricultural land in
hilly areas of the island of Lesvos. In more detail, the objective of the present work is
Ža. to evaluate the effect of land abandonment for a long period on soil properties and
vegetation establishment in hilly areas cultivated for centuries with rain-fed cereals
olives and vines and Žb. to define critical soil characteristics for changing land use from
arable to abandoned pasture for soils formed on different parent materials so that
regeneration of natural vegetation under given climatic conditions can be monitored.
54
C. Kosmas et al.r Catena 40 (2000) 51–68
2. Materials and methods
2.1. Description of the study area
The study was conducted at various sites throughout Lesvos. This island is located in
the northeast part of the Aegean sea and covers an area of 163,429 hectares. The soils of
the study sites were previously cultivated, mainly with rain-fed crops such as cereals,
vines and olives, and due to low productivity the majority of sites were abandoned
40–45 years ago. After abandonment, the area was moderately grazed and the growing
shrubs were occasionally cleared by using fire. The study sites were cultivated using
animal labour once Žcereals. or twice Žvines, olives. per year before abandonment. No
pesticides were used and the only fertiliser applied was animal manure. The present
cultivation practice in the cultivated areas has not significantly changed in the study
sites. The land is mainly cultivated with animal labour or small two-wheel tractors. Full
mechanisation of cultivation is not easy in the area due to steep slopes and the presence
of narrow terraces restricting the use of heavy machinery. The only significant recent
change is the application of inorganic fertilisers in combination with manure. Use of
pesticides is greatly restricted to protect grazing animals from poisoning. Therefore, the
soil state at the time of abandonment was different under the various crops, but after a
period of 40–45 years under natural vegetation and moderate grazing, an equilibrium
under the prevailing soil and climatic conditions of the area has probably been attained.
The study sites are located in hilly areas with slopes ranging from 18–25% for
abandoned sites and from 14–21% for cultivated sites. The soils of the study sites are
usually shallow Ždepth ranging from 5 to 30 cm. to moderately deep Ždepth 30–65 cm.,
well-drained, slightly stony to stony Žrock fragments content 5–25%., sandy loam to
sandy clay loam, developed on various lithological formations such as shale, schist-marble, volcanic lava, pyroclastics, and ignimbrite. They are classified as Typic Xerochrept,
Lithic Xerochrept, or Lithic Xerorthent ŽSoil Survey Staff, 1975..
The climate of the study sites is characterised by strong seasonal and spatial
variations of rainfall and high oscillations between minimum and maximum daily
temperatures, typical of the Mediterranean region. A gradient in rainfall occurs across
the island with the average annual rainfall in the study sites ranging from 677 mm
Žeastern part. to 415 mm Žwestern part.. The mean annual air temperature is 17.78C
without any significant gradient across the island.
In the study sites, the dominant natural vegetation growing after abandonment
belongs mainly to the family Gramineae Ž55–62% of the existing species.. Common
species of this family are: AÕena barbata, Briza maxima, Aegilops neglata, Briza
maxima, Bromus intermedius, Hordeum murinum, Phalaris canariensis, Bromus diandrus, and Lagurus oÕatus. 15–21% of species belong to the Compositae. The following
species are common: Anthemis arÕensis, Carduus nutans, Anthemis cretica, Crepis
rudra, Filago sp., Lactuca seriola, and Thagopogon porrifolius. The remaining common plant species belong to families such as Cruciferae, Labiatae, Leguminosae
ŽTrifolium sp.., and Rosaceae. Perennial shrubs of Sarcopoterium spinosum, and
Sarcopoterium Õerucosum are wide-spread in the area, covering much of the land
surface. The vegetation cover has been affected by the parent material ranging from bare
C. Kosmas et al.r Catena 40 (2000) 51–68
55
to fully covered Ž100%. ŽTable 1.. Vegetation cover was generally lower for soils
formed on pyroclastics Žmean 50.5%, Table 1., while the highest mean vegetation cover
Ž81.4%. was measured in soils formed on shale. Vegetation cover had intermediate
mean values in sites with soils formed on schist-marble, lava, and ignimbrite ŽTable 1..
2.2. Field measurements and sampling
A total of 106 sites were selected for soil analysis along catenas in hilly areas ŽFig.
1.. The selected sites were located on soils formed on igneous rocks such as lava Ž24
sites., pyroclastics Ž17 sites., and ignimbrite Ž17 sites.; sedimentary and metamorphic
rocks such as shale Ž24 sites., and schist-marble Ž24 sites., respectively. The study sites
were located along north-facing slopes on a distinct landscape position which was
identified as backslope, classified according to the scheme of Ruhe Ž1960. scheme. At
each study site, a transect 24 m long was defined perpendicular to the contour lines and
12 disturbed soil samples were collected at an equal distance of 2 m and a composite
Table 1
Vegetation cover Ž%. measured at the various study sites classified according to the parent material
Study site
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Mean
Standard deviation
Parent material
Pyroclastics
Lava
Schist-marble
Shale
Ignimbrite
84.5
75.2
100
5.6
0
0
83.1
52.3
55.1
65.0
45.0
83.2
62.5
53.5
72.1
64.3
62.7
74.2
15.8
43.0
53.5
55.8
0
15.3
50.5
29.6
73.1
100
71.5
84.0
100
95.7
100
94.1
97.2
8.9
82.3
100
87.6
0
53.3
82.2
100
92.1
57.7
24.3
47.0
82.2
5.8
35.2
69.7
32.9
85.7
55.3
84.3
83.8
62.3
65.1
92.3
84.5
93.1
68.9
55.0
85.4
98.1
95.2
93.1
15.3
5.2
0
35.6
25.9
18.1
17.6
84.3
92.2
62.3
32.7
65.9
97.9
72.4
85.7
87.2
96.7
100
88.9
75.8
73.2
95.0
100
55.8
85.6
74.2
70.2
87.8
95.5
99.0
87.4
64.6
74.9
54.6
65.8
81.5
14.2
75.9
40.6
85.9
20.4
82.7
100
100
72.1
81.9
70.5
71.3
73.9
68.3
100
74.2
88.9
95.2
92.4
60.8
65.2
92.1
35.4
0
40.1
70.3
26.1
56
C. Kosmas et al.r Catena 40 (2000) 51–68
Fig. 1. Location and distribution of the study sites on the island of Lesvos.
C. Kosmas et al.r Catena 40 (2000) 51–68
57
sample was prepared. Three separate undisturbed soil samples were also collected at
each site. Soil samples were taken mainly from the A-horizon because major alterations
have occurred in this horizon after the land use change. At each study site, the following
measurements were conducted: dominant plant species, percentage vegetation cover,
shrub diameter, soil depth, thickness of the A-horizon, and slope gradient. Profile or soil
surface stoniness was not determined because surface rock fragment distribution on a
hillslope is greatly affected by animal trampling along favoured routes ŽOostwoud
Wijdenes et al., 1998.. For a comparative analysis of the effect of land abandonment on
soil properties, 23 of the sites were located in cultivated fields. Study sites in cultivated
fields for soils formed on pyroclastics and ignimbrite could not be included due to the
absence of cultivated fields in the area during this study.
The degree of erosion Žno erosion, slightly eroded, moderately eroded, severely
eroded, and very severely eroded. was estimated by measuring the length of visually
current distinct erosion spots and rock outcrops along the study transects. At each study
site, three transects of 30 m each were located perpendicular to the contour lines and the
length of distinct erosion spots and positions of the rock outcrops were measured. The
degree of erosion was then assessed as the ratio of the length of erosion spots and rock
outcrops over the total length of the study transect expressed on a percentage basis
ŽKosmas et al., 1998.. In addition, the rate of exposure of the bedrock was measured at
12 sites in areas overlying pyroclastics. Stainless steel pins were permanently installed
into the bedrock defining the exact size of the erosion spots Žarea of spots ranging from
0.8 m2 to 22 m2 . where the soil had washed out in the past and bedrock has been
exposed at the surface.
Meteorological data were collected from two stations installed at Vatousa and
Antissa. Precipitation was measured via an automatic rain gauge at intervals of 5 min.
Open pan-evaporation rate Ž Eo . was automatically recorded every 6 h. Surface water
runoff was recorded at two plots Žarea 20 m2 . automatically by using dipping buckets.
All data were recorded on Campbell CR10 data loggers.
2.3. Laboratory measurements
The selected soil samples were analysed for properties that affect nutrient and water
availability to the plants and erosion resistance. The particle-size distribution of the
- 2-mm fraction was determined by the Bouyoucos hydrometer method ŽGee and
Bauder, 1986.. The organic carbon content was measured using the modified Walkley–
Black wet oxidation procedure ŽNelson and Sommers, 1982.. pH was measured in a
soilrwater ratio 1:1 ŽMcLean, 1982.. CEC was measured after extraction with ammonium acetate ŽRhoades, 1982.. Exchangeable sodium and potassium were determined
after extraction with ammonium acetate ŽpH s 8.5. ŽThomas, 1982.. Water retention at
y33 kPa was measured in undisturbed soil samples and at y1500 kPa in disturbed
samples. The volume of available water to plants was then determined as the difference
between water retained at y33 and at y1500 kPa ŽKlute, 1986.. The wet sieving
technique of Yoder Ž1936. was used for the determination of the mean soil weight
diameter of the soil aggregates.
58
C. Kosmas et al.r Catena 40 (2000) 51–68
3. Results and discussion
3.1. Land use change effects on soil properties
Textural analysis showed that soils formed on ignimbrite had the highest average clay
content Ž20.0%, standard deviation, SD s 9.7%.. Soils formed on volcanic lava and
pyroclastics had slightly lower clay contents Ž17.8%, SD s 5.9%; 17.1%, SD s 6.3%,
respectively. than soils formed on ignimbrite. The lowest clay content was measured in
soils formed on schist-marble Ž12.4%, SD s 5.3%.. A multiple comparison analysis of
the clay content among the soils formed on various parent materials showed that only
soils formed on schist-marble are significantly different Žat the 0.05 level.. The highest
sand content was measured in soils formed on pyroclastics Ž63.7%, SD s 6.6%., while
on other parent materials sand content averaged 49.7% to 55.5%. As was expected, no
statistically significant differences have been detected between the abandoned and
cultivated soils studied. While no great ranges in clay and sand content have been found
in the various study sites, the present vegetation cover was greatly affected by the parent
material ŽTable 1.. Soils formed on shale had generally higher vegetation cover as
compared to soils formed on pyroclastics.
The soils formed on schist-marble were moderately alkaline Žaverage pH s 7.2.,
while all the other soils were slightly acid ŽTable 2.. The acidity of the cultivated soils
was slightly lower than that of the abandoned soils formed on the same parent materials,
but not statistically significantly different at the 0.05 level. This is mainly attributed to
the distribution of the cultivated soils in the landscape. They are usually found at lower
altitude favouring cation enrichment by overland or sub-surface water flow from the
surrounding areas.
The abandonment of the land has greatly increased soil organic matter content and
soil aggregate stability, especially for the soils formed on lava and schist-marble, as
compared to the soils under cultivation ŽTable 2.. The greatest change in organic matter
content was measured in soils formed on volcanic lava. As Table 2 shows, the cultivated
soils formed on lava had the lowest organic matter content Ž0.92%. under the existing
cultivation practices and climatic conditions of the study area. This may be attributed to
frequent disturbance of these soils by occasionally removing rock fragments and
boulders to aid mechanical cultivation. The removal of boulders from the fields is
mainly achieved by using bulldozers resulting in mixing of the A-horizon with subsurface soil materials.
Soils formed on shale did not show significant differences in organic matter content
after abandonment ŽTable 2.. This may be due to the physical structure and mineralogy
of the bedrock. The existence of easily weathered, layered silicates, feldspars, mica, and
hydrated mica favours the partial weathering of the consolidated rock and the adsorption
of water, permitting root penetration from the growing plants along fracture planes of
the bedrock. Therefore, plant roots can penetrate deeper than just through the shallow
soil, and the organic matter is more uniformly distributed in deeper layers. Measurements of organic matter in sub-surface layers of selected sites showed that the highest
content was measured in soils formed on shale Žmean s 0.78%., followed by soils
Table 2
Mean and standard deviation ŽSD. of soil properties measured in abandoned or cultivated areas with soil formed on various parent materials
Soil Property
Land use
Soil formed on
pH
Organic matter Ž%.
CEC Žcmolrkg.
Exchangeable Na
Exchangeable K
Aggregate size Žmm.
a
abandoned
cultivated
abandoned
cultivated
abandoned
cultivated
abandoned
cultivated
abandoned
cultivated
abandoned
cultivated
Schist-marble
Shale
n c s16 a
Žmean.
n a s8
ŽSD.
n c s17
Žmean.
na s 7
ŽSD.
n c s16
Žmean.
n a s8
ŽSD.
Ignimbrite
6.0
6.12
2.36
0.92
14.1
15.5
48.5
60.2
218.7
277.5
6.70
1.40
1.52
0.71
1.19
0.48
8.8
5.6
15.3
27.1
105.8
70.5
1.10
0.90
7.23
7.68
3.00
1.87
12.7
14.1
41.8
65.1
127.2
265.5
5.70
0.50
1.29
0.59
1.10
0.97
10.6
3.40
9.40
16.5
52.9
119.1
3.30
0.50
6.38
6.80
2.39
2.25
19.3
16.6
29.1
42.9
128.6
177.7
5.90
0.80
1.53
1.29
1.76
1.01
4.10
3.80
9.40
10.5
48.5
79.4
2.80
0.50
n c s17
Žmean.
Pyroclastics
ŽSD.
n c s17
Žmean.
ŽSD.
6.12
0.94
6.14
0.82
2.87
1.01
2.10
0.57
19.9
6.70
16.1
7.30
57.3
24.7
73.9
17.6
219.4
61.7
185.4
64.5
6.6
2.2
3.00
C. Kosmas et al.r Catena 40 (2000) 51–68
Lava
2.10
n c , n a s number of measurements in abandoned and cultivated sites.
59
60
C. Kosmas et al.r Catena 40 (2000) 51–68
formed on schist-marble Ž0.65%., ignimbrite Ž0.54%., pyroclastics Ž0.45%., and lava
Ž0.42%..
Soils formed on shale have the greatest ability to support perennial or annual
vegetation compared to the other parent materials studied in the area due to their high
WHC ŽFig. 2. and water storage capacity in the dense network of fracture planes in the
bedrock. The higher potential of these soils to support natural vegetation or agricultural
crops resulted in relatively small differences in the average values of organic matter
content in abandoned and cultivated soils under the prevailing climatic conditions of the
area. Field observations of root distribution in soil profiles showed that roots of plants
growing in soils formed on lava, ignimbrite, and pyroclastics are highly restricted in the
existing shallow soil, significantly increasing the organic matter content in the A-horizon
after abandonment. Root growth into these parent materials is sometimes favoured by
existing cracks or dikes but they do not penetrate the consolidated, hardly weathered,
parent material.
All the soils under cultivation had an average aggregate size ranging from 0.5 mm to
1.4 mm, independent of the parent material. Abandoned soils formed on pyroclastics had
the lowest aggregate size Ž3.0 mm, Table 2.. Soils formed on the other parent materials
had higher average values of aggregate size in the A-horizon, ranging from 5.7 to 6.7
mm ŽTable 2.. Therefore, aggregate stability was greatly affected by the abandonment of
arable land.
Erosion data indicated that soils formed on pyroclastics usually had the highest
degree of erosion, in agreement with the aggregate stability data. As Fig. 3 shows, 84%
of the study soils formed on pyroclastics were severely to very severely eroded. In
Fig. 2. Mean values of WHC of soils formed on various parent materials.
C. Kosmas et al.r Catena 40 (2000) 51–68
61
Fig. 3. Distribution of the various degrees of erosion assessed for sites on different parent materials
ŽWE sslight erosion, MEs moderate erosion, SE ssevere erosion, VSEs very severe erosion..
contrast, the majority of the soils formed on the other parent materials were moderately
eroded ŽFig. 3.. Of course, other parameters such as vegetation, topography, and
intensity of land use have affected soil erosion, affecting the results in Fig. 3.
The difference in CEC was not statistically significant Žat the 0.05 level. between the
abandoned and cultivated soils formed on the same parent material ŽTable 2.. The
moderate increase in organic matter content measured in the abandoned soils did not
significantly affect CEC. Abandoned soils formed on ignimbrite rocks or shale deposits
ŽTable 2. had the highest values of CEC in the A-horizon Ž19.9 and 19.3 cmolrkg soil,
respectively.. Soils formed on volcanic lava, pyroclastics and schist-marble had lower
average values, ranging from 12.7 to 16.3 cmolrkg soil.
Exchangeable Na can affect aggregate stability and therefore soil erodibility ŽImeson,
1995.. The obtained data indicated that exchangeable sodium was lower in the abandoned soils than in soils under cultivation formed on the same parent material ŽTable 2..
The highest average amount of exchangeable Na was measured in soils formed on
pyroclastics. This can be attributed to the restricted leaching of cations occurring in
these soils due to the almost impermeable underlying bedrock. In all cases, the amount
of exchangeable Na was less than 3% of the CEC. This value is not expected to have
any significant effect on clay dispersion and reduction of aggregate stability ŽHillel,
1980.. As Lavee et al. Ž1991., and Imeson Ž1995. pointed out, below a rainfall of about
300 mmryear, soil degradation is related more to the dynamics of sodium than to
organic matter content, while organic matter dynamics dominate over sodium effects
where annual rainfall exceeds about 600 mm. The obtained data indicated that organic
matter content dynamics are more important in the study soils than sodium content,
under the climatic conditions of the study area.
62
C. Kosmas et al.r Catena 40 (2000) 51–68
Similar trends to sodium were found for exchangeable potassium. Generally, cultivated soils had higher amounts of K as compared to the corresponding abandoned soils
ŽTable 2. due to the addition of K-fertilisers. Cultivated soils formed on schist-marble
contained the highest amount of K.
3.2. Threshold soil characteristics for changing land use
Semi-arid landscapes such as this study area are water-limited by definition and
therefore are potentially sensitive to environmental change and to reduced plant growth.
The potential evapotranspiration ŽETo., estimated by meteorological data measured at
two stations, Vatousa and Antissa, was 1206 and 1533 mm during the period from May
1996 to May 1997, while the total precipitation during that period was 502 and 526 mm,
respectively, indicating a high water deficit for the growing plants in the study area.
The water storage capacity of a soil is defined by the WHC of each soil layer.
According to the average values of WHC, the study soils can be classified into the
following two groups: Ža. soils formed on pyroclastics, lava, and ignimbrite with a
relatively low WHC Ž13.1–14.8 mm of water per 10 cm soil, Fig. 2., and Žb. soils
formed on schist-marble, and shale with a relatively high WHC Ž22.1–22.7 mm of water
per 10 cm soil.. The presence of cracks in the bedrock of lava, ignimbrite, schist-marble
and fractures in shale increases soil water storage capacity allowing the plant roots to
penetrate them and to absorb the water. The efficiency of bedrock to support vegetation
with water stored in cracks or fractures depends on the extent to which these features are
present into the bedrock. In addition, the presence of rock fragments on the soil surface
in these soils restricts soil water evaporation. Therefore, the establishment of natural
vegetation in the study area after abandonment was highly favoured by soil properties
increasing water storage capacity such as parent material, soil depth, stoniness, presence
of cracks, and degree of bedrock weathering.
As Fig. 4 shows, there is a minimum soil depth, depending on the parent material,
below which no shrubby vegetation can be grown under the prevailing climatic
conditions in the study area. The diameter of shrubs, such as Sarcopoterium sp., was
reduced as the soil depth was diminished down to a minimum value at a full stage of
plant development. The rate of reduction in diameter accompanied by reduction in
vegetation cover of these soils was greater on soils formed on pyroclastics than on the
other parent materials. Under the existing soil and climatic conditions, a minimum
diameter of about 24–25 cm of the growing shrubs of Sarcopoterium has been measured
in the study sites ŽFig. 4.. Therefore, no data appear in Fig. 4 for Sarcopoterium of
diameters less than 25 cm.
In this study, two soil depths very important for land protection have been distinguished, the critical and the crucial depth. The critical depth has been defined as the
depth below which the recovery of the natural perennial vegetation is very low and
where the erosional processes may be very active, resulting in further degradation and
desertification of the land. Crucial depth has been defined as the soil depth below which
perennial vegetation cannot be supported and where the remaining soil is rapidly washed
away by wind or water erosion. The importance of these two depths for land protection
becomes apparent in the following discussion.
C. Kosmas et al.r Catena 40 (2000) 51–68
63
Fig. 4. Relationship of the average diameter of Sarcopoterium sp. shrubs and soil depth measured in
abandoned soils formed on pyroclastics.
As Fig. 5 shows, there is a crucial minimum depth of 10 cm for soils formed on
pyroclastics for perennial vegetation to grow in the area. In contrast, soils formed on
Fig. 5. Relationship of percentage vegetation cover of Sarcopoterium sp. shrubs and soil depth measured in
areas with soils formed on pyroclastics and schist-marble.
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C. Kosmas et al.r Catena 40 (2000) 51–68
Fig. 6. Relationship of percentage vegetation cover of Sarcopoterium sp. shrubs and soil depth measured in
areas with soils formed on volcanic lava and ignimbrite.
schist-marble, lava and ignimbrite have a greater vegetation cover for the same soil
depth ŽFigs. 5 and 6.. The measured crucial soil depth for these soils was around 4–5
cm. The rate of the area of increase of plant cover with increasing soil depth was higher
in these soils than in the soils formed on pyroclastics.
If soil depth is less than 10 cm in landscapes with pyroclastics, then only some
annual plant species can survive under the prevailing climatic conditions of this area
ŽFig. 7. and erosion is high. Twelve runoff events have been measured in the study sites
with pyroclastics ranging from 0.1 to 3.4 mm during the period from May 1997 to May
1998. The area of bedrock exposed in the study spots during that runoff events ranged
from 3.3 to 12.5% of the initial area. Soil has been washed out by a combination of
sub-surface Žabove impermeable bedrock. and surface water flow and the bedrock has
been exposed on the surface.
In addition to the crucial soil depth of 4–10 cm, the critical soil depth is very
important for supporting plant cover by perennial shrubs ŽFig. 7.. A value of 40%
vegetation cover is considered critical, below which accelerated erosion dominates on
sloping land ŽThornes, 1988.. If the vegetation cover covers a greater area than 40%,
then it will act as a factor of resilience or protection of the land. The data obtained
shows that areas with soils formed on schist-marbles and having a depth of at least 25
cm had more than 80% plant cover, while areas with soils formed on pyroclastics had
about half of the plant cover for the same soil depth and climatic conditions ŽFig. 5..
Therefore, the nature of the parent material becomes important in vegetation establishment and land protection as soil depth is reduced due to erosion.
C. Kosmas et al.r Catena 40 (2000) 51–68
65
Fig. 7. Abandoned area with soil formed on pyroclastics showing distinct patches with different vegetation
cover due to change in soil depth Žpatch with soil having the critical depth is mainly covered with
Sarcopoterium sp., while patches with soil having the crucial depth are partially covered only with annual
vegetation..
Fig. 8. Relationship of soil depth and thickness of A-horizon developed under Sarcopoterium sp. shrubs in
soils formed on pyroclastics.
66
C. Kosmas et al.r Catena 40 (2000) 51–68
The thickness of the A-horizon in the study area was also related to soil depth. Under
the existing climatic conditions, the thickness of the A-horizon of soils formed on
pyroclastics increased linearly with increasing soil depth until a maximum value of
about 30 cm and then remained almost constant with increasing soil depth ŽFig. 8.. Soils
with a deep A-horizon had a good vegetation cover Žgreater than 65%. and were
relatively well-protected from erosion. The potential for biomass production of these
soils is high and they must be considered as having low risk for degradation under the
present land use and management practice. Similar trends of soil depth and thickness of
the A-horizon were found for soils formed on the other parent materials, with the
maximum thickness of the A-horizon ranging from 28 to 33 cm. It can be inferred from
the above data that a depth of 25–30 cm must be considered as a key indicator for
abandonment of hilly cultivated areas. Under the existing soil and climate conditions of
the study area, this depth can support a natural vegetation cover sufficient to protect the
land. It has to be emphasised that this critical depth can be used for land use planning
and protection of the environment only under the prevailing soil and climate characteristics of the area. Generalisation of this conclusion may be risky for other areas.
4. Conclusions
The following conclusions can be inferred from the obtained data on land abandonment and protection from desertification. The abandonment of the land may lead to
deteriorating or improving conditions of plant cover depending on the soil and climate
characteristics of the area. Soil fertility status may slightly increase after abandonment.
The most significant soil improvement after a period of 40–45 years of abandonment is
related to the increase in organic matter content and aggregate stability of the A-horizon.
Soils formed on pyroclastics have lower capacity to regenerate natural vegetation, and
this leads to higher erosion rates. Soils formed on shale, ignimbrite, schist-marble, and
volcanic lava have a higher capacity for at least partial regeneration of natural
vegetation. The nature of the parent material becomes increasingly important in vegetation establishment and land protection as soil depth is reduced due to erosion. Under the
soil and climatic conditions of the study area, a cultivated hilly landscape must be
abandoned before the soil is depleted to a critical depth of 25–30 cm. The recovery of
the natural vegetation is very low in soils of lesser depth, and erosional processes may
be very active resulting in further degradation and desertification of the land. If a soil is
eroded to a depth of about 10 cm or less, depending on the parent material, then the
perennial vegetation cannot be supported and the remaining soil is rapidly removed by
wind or water erosion. Degradation and desertification of this land is ultimately an
irreversible process.
Acknowledgements
This work was partially financed by the European Union Project ‘‘Mediterranean
Desertification and Land Use’’, MEDALUS III Žcontract number, ENV4 CT95-0119..
C. Kosmas et al.r Catena 40 (2000) 51–68
67
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